Saving jQuery UI Sortable's order to Backbone.js Collection

Question

I have a Backbone.js collection that I would like to be able to sort using jQuery UI's Sortable. Nothing fancy, I just have a list that I would like to be able to sort.

The problem is that I'm not sure how to get the current order of items after being sorted and communicate that to the collection. Sortable can serialize itself, but that won't give me the model data I need to give to the collection.

Ideally, I'd like to be able to just get an array of the current order of the models in the collection and use the reset method for the collection, but I'm not sure how to get the current order. Please share any ideas or examples for getting an array with the current model order.

Answer 1

I've done this by using jQuery UI Sortable to trigger an event on the item view when an item is dropped. I can then trigger another event on the item view that includes the model as data which the collection view is bound to. The collection view can then be responsible for updating the sort order.

Working example

http://jsfiddle.net/7X4PX/260/

jQuery UI Sortable

$(document).ready(function() {
    $('#collection-view').sortable({
        // consider using update instead of stop
        stop: function(event, ui) {
            ui.item.trigger('drop', ui.item.index());
        }
    });
});

The stop event is bound to a function that triggers drop on the DOM node for the item with the item's index (provided by jQuery UI) as data.

Item view

Application.View.Item = Backbone.View.extend({
    tagName: 'li',
    className: 'item-view',
    events: {
        'drop' : 'drop'
    },
    drop: function(event, index) {
        this.$el.trigger('update-sort', [this.model, index]);
    },        
    render: function() {
        $(this.el).html(this.model.get('name') + ' (' + this.model.get('id') + ')');
        return this;
    }
});

The drop event is bound to the drop function which triggers an update-sort event on the item view's DOM node with the data [this.model, index]. That means we are passing the current model and it's index (from jQuery UI sortable) to whomever is bound to the update-sort event.

Items (collection) view

Application.View.Items = Backbone.View.extend({
    events: {
        'update-sort': 'updateSort'
    },
    render: function() {
        this.$el.children().remove();
        this.collection.each(this.appendModelView, this);
        return this;
    },    
    appendModelView: function(model) {
        var el = new Application.View.Item({model: model}).render().el;
        this.$el.append(el);
    },
    updateSort: function(event, model, position) {            
        this.collection.remove(model);

        this.collection.each(function (model, index) {
            var ordinal = index;
            if (index >= position) {
                ordinal += 1;
            }
            model.set('ordinal', ordinal);
        });            

        model.set('ordinal', position);
        this.collection.add(model, {at: position});

        // to update ordinals on server:
        var ids = this.collection.pluck('id');
        $('#post-data').html('post ids to server: ' + ids.join(', '));

        this.render();
    }
}); 

The Items view is bound to the update-sort event and the function uses the data passed by the event (model and index). The model is removed from the collection, the ordinal attribute is updated on each remaining item and the order of items by id is sent to the server to store state.

Collection

Application.Collection.Items = Backbone.Collection.extend({
    model: Application.Model.Item,
    comparator: function(model) {
        return model.get('ordinal');
    },
});

The collection has a comparator function defined which orders the collection by ordinal. This keeps the rendered order of items in sync as the "default order" of the collection is now by the value of the ordinal attribute.

Note there is some duplication of effort: the model doesn't need to be removed and added back to the collection if a collection has a comparator function as the jsfiddle does. Also the view may not need to re-render itself.

Note: compared to the other answer, my feeling was that it was more correct to notify the model instance of the item that it needed to be updated instead of the collection directly. Both approaches are valid. The other answer here goes directly to the collection instead of taking the model-first approach. Pick whichever makes more sense to you.